more hat puzzles  aha:!

i buried four fishermen up to their necks in the sand on the beach at low tide for keeping their fishing spot a secret from me. i put a hat on each of their heads and told them that one of them must shout out the correct color of their own hat or they will all be drowned by the incoming tide. i give them 10 minutes to do this. fisherman A and B can only see the sand dune i erected. fisherman C can see that fisherman B has a white hat on. fisherman D can see that C has a black hat, and B has a white hat. the fisherman have been told that there are four hats, two white and two black, so they know that they must have either a white or a black hat on. who shouts out the color of their hat and how do they know?